How the finite element method helps explaining fatigue crack growth retardation and acceleration

Crack closure effects during fatigue crack growth have been studied by many researchers with the finite element method, but appears difficult to accurately predict. Although quantification of crack closure may be a bridge too far, finite element models may help explaining observations from tests and give insight into trends. This paper studies crack closure resulting from large stress peaks (overloads) and deep valleys (underloads) in a further constant amplitude load. Middle tension and single edge notched specimens of steel and aluminium are simulated. Effects of overloads and of combinations of overloads and underloads for the two geometries and materials are studied and explanations for experimental observations are provided.